Imagine that you’re in the FBI and you receive a tip -- or more likely, pick up information through the kind of mass surveillance in which the national security state now specializes. In a series of tweets, a young man has expressed sympathy for the Islamic State (ISIS), al-Qaeda, or another terrorist group or cause. He’s 16, has no criminal record, and has shown no signs that he might be planning a criminal act. He does, however, seem angry and has demonstrated an interest in following...